2 Answers
2

Retrofitting ground to a range circuit

As of 2014, Code greatly broadened the rules for retrofitting ground wires. If it was simply a matter of provisioning a ground wire to that range circuit, there would be no problem at all.

A 40A circuit requires a #10 ground wire, green or bare. You can always use a larger wire. It only needs to follow the rules for routing and protecting ground wires, and it can follow any route back to the service panel. It doesn't need to go to the panel, only to

any outlet or junction which also has a #10 or larger ground going back to the panel (water heater, A/C, dryer if grounded, etc.

anywhere on the grounding electrode system (the wire to your ground rods) I do not endorse using a general water pipe, because further plumbing mods may separate that.

any junction box connected by all metal conduit back to the panel.

At that point you convert the range receptacle to NEMA 14-50 and change the range plug and jumper, done.

If you were to do this before you converted the circuit to 120V/20A use, there's no question it's legit.

Retrofitting ground to a brand new 20A circuit

This is more fuzzy and I just don't know for sure. I fear it may be subject to interpretation by AHJ. I am concerned that if you had a 3-wire range connection, and then you converted it to 20A receptacles, and then you grounded it, the ruling might be "this is new work; the retrofit rules don't apply to new work; run new homerun(s).”

However if the AHJ is simpatico with this application, then you would be able to retrofit ground to these new receptacle circuits. Same rules as above, however, the size of ground wire you need is 12 AWG (14 AWG if 15A circuit). Obviously this is a lot easier, because you only need to reach a point with a #12 (or #14) ground wire running back to the panel, and that can probably be found a whole lot closer.

And to be clear on this point, larger wire than proscribed is always OK. Suppose the required size is #12... you use #8 ground wire because you have some. You reach a point with #10 running back to the panel. That's all fine for grounding either a 40A or 15/20A circuit.

Breaker Size

The breaker must be sized to protect the wire (so 40A would be fine) but must ALSO be sized to protect the outlet, which is only rated for 20A. So if you are reusing the cable with a 20A outlet then you MUST put in a 20A breaker to match.

Wire Size

I'm no expert on this, but as you pointed out already, there may be a problem getting the 8AWG wire to fit properly with the outlet. Check the specs on the outlet. I just checked one Leviton at random and it is specd. at #10 - #14. If the wire is not compatible, don't connect it directly. Instead use a compatible (again, check the specs) wire nut to connect the 8AWG stranded to a short piece of 12AWG solid wire, and connect the other end of the 12AWG to the outlet. Always use the screws, not the back connections. And black (or red if the other 1/2 of an MWBC)->black, white->white.

Ground

Here is where it gets tricky. Ground is normally green insulation or just a bare wire. It seems pretty clear from this question about ground wiring that you can NOT turn another color (e.g., red) into green except by marking the whole length of the wire. So that rules out (in any practical way) reusing the wire.

However, there are two other possibilities:

If the existing wires are in a metal box attached to metal conduit or armored cable, then you may have a usable grounding path via screwing a ground conductor from the outlet to the metal box.

You can connect a new ground wire to another connection. Grounds do not have to follow the same path as the hot & neutral. This is a relatively new change, and I am sure Harper or one of the other pros will be happy to explain in more detail. But basically, you can connect an appropriately sized ground wire (for sure 12 AWG will do, I'm not sure if there is any allowance for anything smaller) from the outlet to the ground of another outlet in the kitchen.

GFCI

Since this is in the kitchen, you should put in a GFCI. That can be a breaker with GFCI, or it can be an outlet with GFCI. Your old stove was (a) likely put in before GFCI was required and (b) exempt anyway because it was 240V equipment.

MWBC

You can actually turn this 240V connection into TWO 120V circuits using a Multi-Wire Branch Circuit. However, this raises some other potential complications and if all you really need is 20A for the oven ignition and the hood, then it is probably not worth the hassle. But if you had been thinking "another counter outlet would be nice to have" then that is a way to do it using the same 8/3. (Or rather, by actually making use of that red wire instead of capping it and forgetting it.)

Thanks for the reply. I was concerned about using the incorrect color. Since there isn’t a easy solution to ground this circuit (the whole house is ungrounded) I’ll bite the bullet and run a couple of new circuits dedicated to the range and hood with GFCI breakers.
– Roger Houston Nov 9 '18 at 3:28

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I like this answer.i I don't have my code book handy but at #8 wire can you remark the ground? It may be just the neutral, that's it is legal for.+
– Ed BealNov 10 '18 at 0:26

@EdBeal Thank you (and upvote if you haven't already... :-) ) As far as remarking the ground, I linked to a question where Harper answers pretty conclusively that you can't remark the ground except the whole length (which would be pointless).
– manassehkatzNov 10 '18 at 23:35